Vertical steam generator with central downcomber



Nov. 22 1966 c. R; GREEN ETAL VERTICAL STEAM GENERATOR WITH CENTRALDOWNGOMER Filed Oct. 29, 1965 2 Sheets-Sheet 1 INVENTORS RONALD B. CRK04421.55 2.6REEN ATTORNEY Nov. 22, 1966 c. R. GREEN ETAL 3,286,696

VERTICAL STEAM GENERATOR WITH CENTRAL DOWNCOMER "Filed Oct. 29, 1965 2Sheets-$heet 2 -INVENTORS RONALD 5. CREEK BY CHARLES R. GREEN ATTORNEYUnited States Patent 3,286,696 VERTICAL STEAM GENERATOR WITH CENTRALDOWNCOMER Charles R. Green and Ronald B. Creek, Chattanooga, Tenn,assignors to Combustion Engineering, Inc.,

Windsor, Conn., a corporation of Delaware Filed Oct. 29, 1965, Ser. No.505,665

13 Claims. (Cl. 122--34) The present invention relates generally tovapor generators of the shell and tube type wherein heat is supplied bya heating fluid flowing through tubes that are immersed in a vaporizableliquid contained within the shell. More particularly, the inventionrelates to a novel vapor generator design of the aforementioned typehaving means for more effectively utilizing the heating surfaceavailable within the vapor generator in order to achieve greateroperational efliciency and a more compact form.

It has been heretofore known in the art to orient a shell and tube typevapor generator such that its axis extends vertically in order to reducethe amount of floor space required for mounting such units. Vaporgenerators having such an attitude have, of necessity, been constructedwith tube sheets of an extreme thickness in order to overcome stressesimposed upon the tube sheet by its own weight, by the weight of thetubes that it serves to mount and by the difference in fluid pressurethat exists'on either side of the tube sheet during unit operation. Ithas been proposed to reduce the amount of tube sheet thickness requiredin such units by providing a structural member intermediate the ends ofthe tube sheet which extends between it and the Wall of the pressurevessel in order to thereby transfer some of the loading on the tubesheet to the vessel wall. In order to accomplish this it is necessary tomaintain that portion of the tube sheet to which the support is attachedvoid of tube attachments thereby eliminating a wide swath of heatingsurface from service. I

It has also generally been known that for most effective operation ofvapor generators of the shell and tube type it is necessary to definedistinct fluidvflow paths through which the vaporiza'ble liquid flowswhile being circulated through the unit. These paths are conventionallyformed by means of fluid impervious baffles or the like. One of suchflow paths is normally referred to as the riser path, since it is inthis portion of the fluid circuit that the heat exchange tubes arelocated for heating the vaporizable liquid and transforming a portionthereof into steam that rises toward the top of the vessel. The othersuch path, referred to as the downcomer path, is that which is void ofheat exchange tubes and therefore relatively unheated. It is throughthis portion of the circuit that liquid is passed downwardly toward thebottom of the vessel due to the greater density of the fluid that existsin this portion of the circuit. The necessity of eliminating heatexchange tubes from the downcomer path still further reduce the amountof heating surface capable of being provided Within the vapor generatorshell.

By means of the present invention there is provided a vapor generatorwherein means are employed for sup porting the tube sheet in order toreduce its requisite thickness and fluidly distinct flow paths areprovided for directing the recirculating vaporiza'ble fluid through theunit. The tube sheet support and the plate means employed to define thefluid flow paths are so disposed as to reduce to a minimum the amount ofvessel volume that is not available for containing heating surface,there- 'by increasing the over-all efficiency of the unit and reducingthe amount of space necessary for containing the unit. Moreover, thepresent invention provides means for collecting the liquid separatedfrom the vapor-liquid mixture created in the vapor generating portion ofthe unit and for returning it to the downcomer portion of the circuit ina minimum amount of vapor generation space and in a manner whereby itsflow velocity is not unduly increased.

Various other objects and advantages will appear from the followingdescription of one embodiment of the invention, and the novel featureswill be particularly pointed out hereinafter in connection with theappended claims.

The invention is described with reference to the accompanying drawingswherein:

FIGURE 1 is a vertical section of a vapor generator constructed inaccordance with the invention;

FIGURE 2 is a vertical section taken along line 22 of FIGURE 1;

FIGURE 3 is a horizontal section taken along line 3-3 of FIGURE 1; and

FIGURE 4 is a vertical section of a modified form of vapor generator.

Referring now to the drawings the numeral 10 represents a vaporgenerator constructed in accordance with the invention. The vaporgenerator 10 comprises a vertically elongated pressure vessel includinga first cylindrical wall 12 and a second cylindrical wall 14 of greaterdiameter than the first connected thereto by a generally conicaltransition portion 16. The upper and lower ends of the vessel are closedby hemispherical end portion 18 and 20, respectively. A generallyannular tube sheet 22 extends normal to the axis of the vessel adjacentthe lower end thereof dividing the vessel into a vapor generatingsection 24 and a heating fluid section 26. The tube sheet 22 contains aplurality of tube seats for mounting the ends of the tubes 28 that forma tube bundle 30. The tube bundle 30 comprises parallel rows of U-shapedtubes 28 having their bends positioned uppermost in the vessel and theirends attached to the tube sheet 22 in communication with the heatingfluid section 26. Within the vapor generating section 24 is contained abody of liquid in which the tubes 28 of the tube bundle 30 are immersed.The liquid has a level 32, here shown in the position which it occupiesduring normal operating conditions, that divides the vapor generatingsection 24 into a lower liquid space 34 and an upper vapor space 36.Liquid is admitted to the vessel by means of a feedwater nozzle 33attached to the vessel wall 12 and a duct 35 that conducts feedwater tothe downcomer portion of the vessel. A vapor nozzle 38 atached to theend wall portion 18 is provided for removal of vapor created in thegenerator to it point of intended use.

As shown in the drawings, the tube sheet 22 is supported at its centerby an axially disposed cylindrical support member 40 that is attached asby means of Welding at its .top to the lower surface of the tube sheetand at its bottom to the inside surface of the lower end wall portion20. .By supporting the tube sheet 22 in this manner its unsupportedlength is considerably reduced, thereby reducing the amount of thicknessthat would otherwise be necessary to Withstand the stresses imposed bythe weight of the tube sheet, the Weight of the tubes 28 and thedifferential fluid pressure that exist between the fluid contained inthe vapor generating section 24 and that contained in the heating fluidsection 26. As shown, tube sheet 22 and the end wall portion 20 containopenings 42 and 44, respectively, that are aligned with the centralopening through the support cylinder thereby providing a manway 46 forconvenient access by a workman to the vapor generating section 24 formaintenance or replacement of the components contained therein. A coverplate 48 that may be attached to the outer surface of the lower end wallportion 20 as by means of threaded fasteners is provided for coveringthe manway.

Within the heating fluid section 26 are provided a pair of flat,diametrically opposed plates 50 for dividing the section into an inletchamber 52 and an outlet chamber 54. The plates 50 conform generally tothe shape of the heating fluid section 26 and are attached, as by meansof a continuous, fluid impervious weld, to the outside of the tube sheet22, the outer surface of the tube sheet support 40 and to the inner wallof the lower end wall portion 20 to form fluidly distinct chambers.Inlet and outlet nozzles 56 and 58, respectively, are attached to theend wall portion 20 to establish fluid communication between therespective chambers, 52 and 54, and a source of heating fluid (notshown). ,By means of this arrangement the vapor generator is providedwith a continuous source of heating fluid.

The generation of vapor occurs within the vapor generating section 24where the U-tubes 28 that conduct the heating fluid through the unit areimmersed as shown in a body of vaporizable liquid, such as water. Heatcontained in the heating fluid is given up to the cooler vaporizableliquid thereby transforming a portion of the liquid into vapor whichrises toward the vapor space 36 due to thermal siphonic action therebyestablishing fluid circulation through the vapor generator. It isgenerally known that circulation efficiency of a vapor generator of thistype will be increased when the flow paths through the vapor generatingsection 24 are distinctly defined as by means of bafiies that separatethe riser portion of the fluid circuit from the downcomer portionthereof. According to the present invention therefore, there is provideda cylindrical conduit 60 axially disposed within the vessel in spacedrelation from the wall 12 thereof, thereby defining an inner downcomerchamber 62 for conducting liquid to the bottom of the vapor generatingsection 24 and an outer, annular chamber 64 within which the tubes 28are located and which comprises the riser portion of the fluid circuit.As shown the conduit 60 is conveniently located within the tube-freevoid 66 formed at the axial center of the tube bundle 30. This void 66is formed as a result of the fact that the tube bundle is formed ofU-shaped tubes which must inherently provide such a void in the spaceenclosed by the tubes having bends of minimum dimension. The conduit 60is closed at its upper end by a plate 61 and extends substantially tothe bottom of the vapor generating section 24 but, as shown, is somewhatspaced from the upper surface of the tube sheet 22 in order to establishfluid communication between the downcomer chamber 62 and the outerannular chamber 64. The conduit 60 is further constructed of a diameterthat substantially coincides with that of the tube sheet support 40 andis positioned in axial alignment therewith in order that the effectivetube mounting surface of the tube sheet 22 is not reduced below thatamount that is necessary for attaching the tube sheet support 40-.

Other baflie means further provided in the vapor generator 10 include anenlarged annular downcomer trough 68 spaced axially above the tubebundle 30 which, together with baflie plates 70 and 72, form a reversinghood type of vapor separator for removing entrained liquid from themixture that emerges from the vapor generating space 24. As shown thedowncomer trough 68 comprises an enlarged, open, annular receptacleformed by a substantially cylindrical plate 74 concentrically spacedfrom the wall 14 of the vessel as by means of circumferentially spacedbrackets 76 and a substantially conically shaped bottom plate 78 that isadapted to discharge liquid collected therein to the downcomer chamber62 for recirculation through the unit. Discharge from the downcomertrough 68 is accommodated by a laterally elongated opening 80 in thebottom plate 78. Communication between the trough 68 and the downcomerchamber 62 is effected by means of a diametrically elongated,

' rectangular duct 82 that attaches at its upper end to the opening 80in the bottom plate 78 and at its lower end to closure plate 61 tothereby permit passage of liquid from the trough 68 to the downcomerchamber 62. The duct 82 is positioned in a lateral void 84 provided inthe tube bundle 30 by the removal of one or more tube rows.

In keeping with the invention the duct is constructed with as narrow awidth dimension as possible in order to minimize the number of tube rowsthat must be removed for accommodation of the duct. The dimensions ofthe duct 82 are dictated by the desired velocity of the liquid flowingthrough it from the downcomer trough 68. The velocity of the liquid mustnot be so high as to result in the liquid having an excessive pressuredrop while flowing through the duct since such excessive pressure dropwould tend to disrupt the fluid circulation stability of the unit byreducing the effective pressure head available for fluid circulation andthereby disrupt fluid flow through the unit. In order to reduce,therefore, the velocity of the liquid flowing through the duct 82 thecross-sectional area required to provide the desired velocity isdetermined and the duct dimensioned so as to contain suchcross-sectional area With a minimum width dimension. In other words, thelength of the duct is made as long as practicable in order that itswidth may be at a minimum.

In the arrangement disclosed in FIGURES 13 the vapor separatingapparatus indicated generally as 86, is in the form of a reversing hoodformed by the arrangement of plates 70 and 72. Plate 70 is an annularmember that is disposed horizontally across the vapor axis with itsouter peripheral edge attached as by means of a continuous weld, to theinner surface of wall 14. The plate 70 contains a central opening 73from which depends a vertically arranged cylindrical plate 72concentrically spaced from the trough wall 74 and depending below thetop edge thereof in order to create a flow passage having an abruptchange of direction such that any liquid contained in the mixture willbe flung therefrom by the centrifugal force developed in the mixtureflowing through the passage.

The operation of the herein disclosed vapor generator is as follows.Heating fluid from a source is admitted by the heating fluid inletnozzle 56' into the compartment 52 from whence it flows through thetubes 28 of the tube bundle 30 through the vapor generating section 24to the heating fluid outlet compartment 54 and thence it is passed bymeans of the outlet nozzle 58 to the source for reheating. At the sametime vaporizable liquid in the form of water is admitted throughthefeedwater inlet nozzle 33 through the feedwater duct 35 into thedowncomer cham ber 62 from whence it passes to the bottom of the vaporgenerating section 24 to the outer annular chamber 64, therebysubstantially filling the chamber as indicated by the position of theliquid level 32. Heat from the heating fluid is transferred to thevaporizable liquid through the heating surface of the tubes 28 whereby aportion of the liquid is transformed into vapor and circulation due tothe thermal siphonic action generated during the transformation of aportion of the liquid into vapor is initiated. Vapor and liquid mixtureemerges from the liquid space 34 and enters the flow passage createdbetween the vessel wall 14 and the downcomer trough 74 and also betweenthe trough wall 74 and the depending annular plate 72 thereby causingthe mixture to undergo an abrupt change of direction in flowing towardthe vapor space 36. Because of this abrupt change of direction,centrifugal force effects the removal of the liquid entrained in thevapor discharging it into the downcomer trough 68. The separated vaporflows through the opening formed by the cylindrical plate 72 into thevapor space 36 and thence through the outlet nozzle 38 to a point ofintended use. The liquid discharged from the vapor mixture is dischargedfrom the trough 68 through the opening 80 from whence it passes throughthe duct 82 into the downcomer chamber 62 where it mixes with theincoming feedwater to be again circulated through the unit. In flowingthrough the duct 82 the velocity of the liquid is maintainedsufiiciently low due to the dimensioning of the duct in order that unduefluid pressure drop is prevented.

By means of the invention there is therefore provided a novel shell andtube type vapor generator which design achieves maximum unit efficiencyin a minimum amount of space and at a lower cost of manufacture. Theductwork that is necessary in the vapor generating section of the unitfor maximum circulation efficiency is so designed and arranged that itrequires the elimination of a minimum amount of heating surface and thatpermits the provision of a tube sheet support within the unit withoutrequiring the removal of additional heating surface to accommodate thesupport. The presence of the tube sheet support in the manner describedreduces the amount of thickness required for the tube sheet andtherefore materially reduces the cost of unit fabrication.

In FIGURE 4 there is shown a slightly modified form of vapor generatorconstructed in accordance with the invention. In this arrangement thevapor separating apparatus, here indicated as 86, takes the form of aplurality of centrifugal separators 88 of known construction, in whichthe vapor and liquid mixture is spun in order to separate the liquidfrom the mixture and to discharge it downwardly into the downcomertrough 68. In the arrangement shown the separators 88 are mounted uponan annular plate 90 that is attached between the top of the cylindricaltrough wall 74 and the inner surface of the vessel wall 14. Mounting theseparators 88 is accomplished by means of an inlet tube 92 attached tothe plate 90 and to the inlet end of the separators.

The operation of this form of vapor generator is substantially the sameas that described above with the exception that instead of the steamflowing through a tortuous path in order to effect gravity separation ofthe liquid it is passed through the separators 88 wherein the bulk ofthe liquid is removed by spinning action that occurs therewithin.

It will be understood that various changes in the details, materials,and arrangements of parts which have been herein described andillustrated in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

What is claimed is:

1. A vapor generator comprising: wall means forming a verticallyelongated pressure vessel of circular cross section; a tube sheetdividing said vessel into a vapor generating section and a heating fluidsection; a body of liquid in said vapor generating section having alevel defining an upper vapor space and a lower liquid space; a vaporoutlet from said vapor space; a tube bundle including parallel rows ofvertically elongated U-tubes immersed in said body of liquid, said tubesbeing disposed with their bends uppermost and with their ends attachedto said tube sheet in communication with said heating fluid section;means for passing a heating fluid through said tubes; means dividingsaid heating fluid section into inlet and outlet portions; axiallydisposed conduit means spaced inwardly from said vessel wall to form aninner downcomer chamber and an outer annular chamber containing saidrows of tubes, said conduit means having its lower end spaced from saidtube sheet to establish communication between said inner and outerchambers; separator means communicating with said outer annular chamberoperative to discharge vapor to said vapor space and liquid to saidliquid space; a downcomer trough positioned in said vapor space andbeing adapted to receive liquid discharged from said separator means;and a vertically elongated, generally rectangular duct disposed betweenadjacent rows of said U-tubes, having its upper end communicating withsaid trough and its lower end communicating with said downcomer chamber,said duct being of sufficient width to provide sufficientcross-sectional area for limiting the velocity of liquid flowingtherethrough.

2. A vapor generator as recited in claim 1 wherein said downcomer troughincludes: an annular wall spaced inwardly from said vessel Wall and agenerally conical bottom open to said duct.

3. A vapor generator as recited in claim 2 including: annular platemeans disposed above said downcomer trough having its outer peripheraledge attached to said vessel wall; and cylindrical plate means dependingfrom the inner peripheral edge of said annular plate means and beingtelescopically received within said annular wall to form a reversinghood separator.

4. A vapor generator as recited in claim 2 including: annular platemeans having its outer peripheral edge attached to said vessel wall andits inner peripheral edge attached to the top of said annular wall; andcentrifugal separator means mounted upon said annular plate means.

5. A vapor generator as recited in claim 4 wherein said centrifugalseparator means includes a plurality of centrifugal separators spacedabout the circumference of said annular plate means.

6. A vapor generator as recited in claim 1 wherein said rectangular ductextends substantially diametrically transverse said pressure vessel andsaid vapor generator further includes: axially disposed tube sheetsupport means supportingly attached between said tube sheet and the Wallforming the bottom of said pressure vessel; and generally flat platemeans cooperating with said tube sheet, said tube sheet support meansand said pressure vessel wall to divide said heating fluid section intoinlet and outlet portions, said flat plate means including a pair ofdiametrically opposed plates in substantial vertical alignment with saidrectangular duct.

7. A vapor generator as recited in claim 6 wherein said tube sheetsupport means comprises an elongated structural cylinder.

8. A vapor generator as recited in claim 7 wherein said axially disposedconduit means is cylindrically formed and said tube sheet support meansis located in axial alignment therewith.

9. A vapor generator as recited in claim 6 wherein said tube sheetsupport means comprises a structural member having cross-sectionaldimensions that are substantially coincident with the cross-sectionaldimensions of said conduit means and in axial alignment therewith.

10. A vapor generator as recited in claim 6 wherein said downcomertrough includes: an annular wall spaced inwardly from said vessel walland a generally conical bottom open to said duct.

11. A vapor generator as recited in claim 10 including: annular platemeans disposed above said downcomer trough having its outer peripheraledge attached to said vessel wall; and cylindrical plate means dependingfrom the inner peripheral edge of said annular plate means and beingtelescopically received within said annular wall to form a reversinghood separator.

12. A vapor generator as recited in claim 10 including: annular platemeans having its outer peripheral edge attached to said vessel wall andits inner peripheral edge attached to the top of said annular wall; andcentrifugal separator means mounted upon said annular plate means.

13. A vapor generator as recited in claim 12 wherein said centrifugalseparator means includes a plurality of centrifugal separators spacedabout the circumferences of said annular plate means.

References Cited by the Examiner UNITED STATES PATENTS 3,071,119 1/1963Ammon et al. l2234 3,114,353 12/1963 Sprague 12234 3,129,697 4/1964Trepaud 12234 KENNETH w. SPRAGUE, Primary Examiner.

1. A VAPOR GENERATOR COMPRISING: WALL MEANS FORMING A VERTICALLYELONGATED PRESSURE VESSEL OF CIRCULAR CROSS SECTION; A TUBE SHEETDIVIDING SAID VESSEL INTO A VAPOR GENERATING SECTION AND HEATING FLUIDSECTION; A BODY OF LIQUID IN SAID VAPOR GENERATING SECTION HAVING ALEVEL DEFINING AN UPPER VAPOR SPACE AND A LOWER LIQUID SPACE; A VAPOROUTLET FROM SAID VAPOR SPACE; A TUBE BUNDLE INCLUDING PARALLEL ROWS OFVERTICALLY ELONGATED U-TUBES IMMERSED IN SAID BODY OF LIQUID, SAID TUBESBEING DISPOSED WITH THEIR BENDS UPPERMOST AND WITH THEIR ENDS ATTACHEDTO SAID TUBE SHEET IN COMMUNICATION WITH SAID HEATING FLUID SECTION;MEANS FOR PASSING A HEATING FLUID THROUGH SAID TUBES; MEANS DIVIDINGSAID HEATING FLUID SECTION INTO INLET AND OUTLET PORTIONS; AXIALLYDISPOSED CONDUIT MEANS SPACED INWARDLY FROM SAID VESSEL WALL TO FORM ANINNER DOWNCOMER CHAMBER AND AN OUTER ANNULAR CHAMBER CONTAINING SAIDROWS OF TUBES, SAID CONDUIT MEANS HAVING ITS LOWER END SPACED FROM SAIDTUBE SHEET TO ESTABLISH COMMUNICATION BETWEEN SAID INNER AND OUTERCHMBERS; SEPARATOR MEANS COMMUNICATING WITH SAID OUTER ANNULAR CHAMBEROPERATIVE TO DISCHARGE VAPOR TO SAID VAPOR SPACE AND LIQUID TO SAIDLIQUID SPACE; A DOWNCOMER TROUGH POSITIONED IN SAID VAPOR SPACE ANDBEING ADAPTED TO RECEIVE LIQUID DISCHARGED FROM SAID SEPARATOR MEANS,AND A VERTICALLY ELONGATED, GENERALLY RECTANGULAR DUCT DISPOSED BETWEENADJACENT ROWS OF SAID U-TUBES, HAVING ITS UPPER END COMMUNICATING WITHSAID TROUGH AND ITS LOWER END COMMUNICATING WITH SAID DOWNCOMER CHAMBER,SAID DUCT BEING OF SUFFICIENT WIDTH TO PROVIDE SUFFICIENTCROSS-SECTIONAL AREA FOR LIMITING THE VELOCITY OF LIQUID FLOWINGTHERETHROUGH.